doc: Emphasize that security issues must not be reported as normal bugs
doc: Emphasize that security issues must not be reported as normal bugs

Change-Id: I43e452c9993a8608b20ec029562982f5dcf8e6b2
Signed-off-by: Sandrine Bailleux <4b69fcfca8da45b0c11ecda5434c5da526575d9d@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware